Consider the traps in the following lyrics:
Brightly beams our Father's mercy
From his lighthouse evermore,
But to us he gives the keeping
Of the lights along the shore.
In the first line, the there are three "ee" vowels, two of which are on unaccented syllables. Then there are three uses of "r" which can swallow the vowel placement into the throat. The diphthong in "bright" and the "ah" in "Father's" can be placed too far back in the throat to match the forward brightness of the "ee" vowels. Equalizing the vowels in just this first line can be a huge task for a singer, let alone an entire choir!
When working with a choir, the director will want to instruct the singers to listen and make adjustments and modifications to help the sound of the choir as a whole. These little adjustments help the overall intonation of the group. They will sound better in tune, better together, and their message will come through more beautifully. Remember to equalize those vowels!